Conventionally, hydrodynamic bearing devices having a thrust dynamic pressure generating portion formed on a surface of either a sleeve or a shaft have been used. In the thrust dynamic pressure generating portion, a dynamic pressure is generated by rotating the sleeve or the shaft which is a rotating part to rotate the rotating part in a non-contact state, which means that a predetermined space is interposed therebetween.
For example, Japanese Laid-Open Publication No. 2000-346056 discloses a thrust dynamic pressure bearing having a plurality of dynamic pressure generating grooves formed on an end surface of a flange formed on an end surface of the shaft. The dynamic pressure generating grooves having a herringbone pattern, and are formed with a predetermined gap interposed among them in a circumferential direction.
The dynamic pressure generating grooves formed in the thrust dynamic pressure bearing gradually become shallower from the outermost periphery toward a central portion in a radial direction, and also gradually become shallower from the innermost periphery toward the central portion in the radial direction. A depth of the grooves in the outermost peripheral portion in the radial direction is greater than a depth of the grooves in the innermost peripheral portion in the radial direction.
Japanese Laid-Open Publication No. 9-303381 discloses a thrust dynamic pressure bearing having a plurality of dynamic pressure grooves for axial supporting arranged in a spiral pattern in a circumferential direction on an end surface of an axis in an axial direction. The outer edge of the end surface has a curved shape.
In the thrust dynamic pressure bearing, a flat portion having a width of 1 mm or shorter, ore preferably, 0.5 mm or shorter is provided between the outer edge and the dynamic pressure grooves.